1. Field of the Invention
The present invention relates to a support structure for a ceramic valve assembly and is used as, for example, a support structure for supporting an exhaust control valve constructed of ceramic from a steel exhaust pipe of an internal combustion engine provided with a turbocharger.
2. Description of the Related Art
Internal combustion engines have included a twin turbocharger system, wherein two turbochargers are provided and an intake control valve and an exhaust control valve are installed in an intake conduit and an exhaust conduit, respectively, of one of the two turbochargers. This allows the turbocharger operation to be switched between a one-turbocharger-operation and a two-turbocharger-operation by opening and closing the intake and exhaust control valves. In these engines, the exhaust control valves are preferably constructed of ceramic to improve the valve's thermal durability, operation reliability, such as sticking prevention, and response characteristic through reduction of gas leakage.
However, since there is a difference in thermal expansion between the ceramic and the exhaust pipe material, a support structure of the exhaust control valve, i.e., an effective ceramic butterfly valve assembly from the exhaust pipe is needed. If the clearance is too small, the clearance cannot absorb the thermal expansion and an excessively large force will act on the ceramic member. Contrarily, if the clearance is too large, the ceramic valve assembly will cause vibration and may be broken due to collision with the metal member.
Japanese Utility Model Publication SHO 55-175663 proposes a support structure for a ceramic butterfly valve assembly, wherein a ceramic liner is attached to an inside surface of the annular metal housing and a ceramic butterfly valve through which a valve shaft penetrates, is disposed inside the ceramic liner so as to be rotatable about an axis of the valve shaft. However, if a very high temperature is applied to the structure, such as in the case of an exhaust control valve of an internal combustion engine with a turbocharger, the ceramic liner will be broken due to the thermal expansion difference between the metal valve housing and the ceramic liner. Further, since the ceramic liner is integrally attached to the valve housing, when leaping stones strike against the valve housing, the ceramic liner may be broken due to the impingement.
Further, in the apparatus of the above-described publication, since a central portion of the ceramic valve through which the valve shaft penetrates is large, a cross-sectional area of an exhaust gas flow passage is reduced and the transient response characteristic of the turbocharger is degraded. Furthermore, since the butterfly valve rotates to a position perpendicular to an axis of the ceramic liner when the valve is closed, foreign substances may engage the clearance between the valve and the liner causing a sticking with the valve housing.